The invention relates to bioreactors used to culture a wide variety of microorganisms and organisms such as algae, for various purposes from filtering dissolved wastes in water, digesting organic wastes, to producing pharmaceutical end-products.
U.S. Pat. Nos. 5,055,186 and 5,593,574 granted Oct. 8, 1991 and Jan. 14, 1997, respectively, to Van Toever, relate to bioreactor systems, primarily biofilter systems, using fluidized pellet media. Although such systems are effective, efforts to scale up the systems have encountered some difficulties, particularly when the objective is to provide a bioreactor system having maximum possible effective surface area for cultural bacteria and other microorganisms to provide a system which is self cleaning and relatively maintenance free as much as possible and to provide a system which operates with low energy consumption.
More particularly, the revolving downflow injector design described in U.S. Pat. No. 5,593,574 works adequately with shallow filter media beds. The concept of fluidizing only a narrow zone of media at any given time rather than the conventional method of continually fluidizing the entire bed of media enabled a drastic decrease in the energy required for fluidization.
Nevertheless, efforts to scale-up such downflow injector filters with greater tank diameters, (greater than ( greater than ) 1 m) and media bed depths, (greater than ( greater than ) 1 m), using this design, required significant increases in pump size to provide sufficient energy to fluidize the pellets. Since the low density plastic pelleted media is buoyant, (specific gravity of 0.91-0.93 relative to water), the downward directed jets of filtrate must have sufficient force to counter the buoyancy and flotation of the media in order to fluidize the bed. With increased bed depth, the energy required increased significantly. By increasing the pressure and flow of filtrate, deep beds could be fluidized but at exceedingly high, if not, prohibitive operating costs.
Additionally, the increased turbulence caused by the high energy injection would frequently cause media pellets to wash out of the filter.
Extensive efforts have lead to the development of a new, much superior configuration.
Initially efforts focused on slowing the rotation of the downflow filtrate injector system represented by U.S. Pat. No. 5,593,574. The fluidization of a given zone is not instantaneous and a period of time is required for the jets of filtrate to penetrate and fluidize a given cross section of media. Efforts to improve the system included the use of low speed gear motors to slow and accurately control the speed of rotation to ensure complete fluidization. With larger beds, that is, with media beds greater than 1 meter in diameter, rotational speeds of xc2xc rpm and filtrate flows of approximately 600/l/min/m2 of filter bed surface area were required. However, faster rotational speeds tended to result in incomplete fluidization of the media. Further, in order for downward directed jets of filtrate to fluidize the media, the jets had to have sufficient energy to counteract the upward flotation, (buoyance), of the media as well as to counteract the friction in the media bed.
Accordingly, it would be advantageous to provide an injector system to fluidize the media bed which would ensure that all areas of the filter media bed receive as uniform a flow of filtrate as possible and which could be expanded radially or indepth to encompass larger media beds.
The earlier U.S. Pat. Nos. 5,055,186 and 5,593,574 referred to above, utilize plastic media pellets and the system to which this invention is directed also depends on the use of plastic media pellets. The purpose of the media is to provide an optimal xe2x80x98engineeredxe2x80x99 surface area for culturing bacteria, fungi and other microorganisms, while at the same time providing the maximum possible effective surface area per unit volume of filter at a reasonable cost. The desired microorganisms require a surface to colonize and with the appropriate nutrients and environment a diverse ecological mix of species establishes and grows to create a biofilm. The biofilm adheres to the substratexe2x80x94media pelletsxe2x80x94and will generally flourish and grow until it plugs the interstitial spaces between the supporting media and blocks the flow of nutrients to the microorganisms. Additionally, particulates in the filtrate also adhere to the xe2x80x9cstickyxe2x80x9d biofilm through a number of mechanisms and serve to accelerate the plugging of the filter. An effective filter therefore has to continually harvest excess biofilm and particulates in order to maintain an optimal biofilm which is constantly in a growth phase condition, rather than one that cycles between xe2x80x9cstart-up-growth-plugging-crashing-cleaning-start-upxe2x80x9d. The fluidized bed design can provide an environment wherein excess biofilm is continually scoured off the media, while sufficient shelter is provided to provide an adequate environment for maintenance of a continually self renewing, optimally, thin biofilm.
Conventional fluidized beds generally utilize randomly configured support media such as sand and plastic material. Creased or grooved media pellets are disclosed in the abovenoted U.S. Patents. Nevertheless, it would be advantageous to have media pellets which have very specific characteristics and which are manufactured to a specific engineered design to optimize film growth and to be compatible with the radial flow injection system developed.
The filter design relies on the buoyancy of the media pellets to maintain the media bed within the filter. Insufficient buoyancy or excessively high filtrate flow rates which result in excess downflow velocities will wash the media out of the filter outlet. Earlier attempts to screen the outlets of the filters proved futile since the biofilm grows rapidly and plugs the screens.
Biofilms for example, have a specific gravity of approximately 1.07 relative to water. The low density plastic pellet has a selected specific gravity in the range of 0.91 to 0.93 so that it floats in water. The media pellet must therefore be designed with sufficient mass so that the ratio of the maximum supportable biofilm mass, to the pellet mass remains less than one or the pellets will sink.
An apparently obvious solution would be to decrease the density of the plastic and increase the buoyancy. A small increase in buoyancy, however leads to drastic increases in the energy required to fluidize the media, especially in the start-up phase when there is no biofilm present to counter the buoyancy of the pellets. Since energy consumption is a critical factor in determining the success of the bioreactor design, significant increases in buoyancy of the media pellets is not a cost effective option.
All characteristics of the pellet must be considered together to achieve a successful design. A balance must be achieved between the cost of materials and manufacturing, the effective surface area for biofilm culture per unit volume of filter and the dimensions of the sheltered grooves which determines the biofilm biomass relative to the mass of plastic per pellet as this relationship determines pellet buoyancy once the biofilm is established. The design of the pellets must be such to minimize interlocking of pellets which increases energy requirements for fluidization. Further, the pellets must be as small as possible to maximize surface area per unit volume while providing adequate mass for buoyancy as described.
Accordingly, it would be advantageous to have pellet media which have proven to be an acceptable compromise between the various design parameters noted above, particularly in fluidized bed systems as set forth herein.
In order to secure greater uniformity in the fluidization of pellets by filtrate, a new approach was investigated wherein the filtrate would be injected horizontally to fluidize the media, as this would eliminate the buoyancy factor. The design developed provides for orifices in a central, vertical rotating, main manifold directing pumped filtrate in horizontal streams or xe2x80x98jetsxe2x80x99 out towards the periphery of the filter bed. Since the main manifold is located in the centre of the cylindrical bed and rotates about the central vertical axis there is virtually no friction to overcome in order to turn it. The central or main manifold rotates slowly enough to permit the jets to horizontally fluidize a vertical zone of media of related narrow arc from the centre extending out to the perimeter of the reactor, a fluidization which is cyclical for the media pellets. With the previous filter design noted in the background of the invention, as the filter depth of the media bed increased, the downward pressure and flow required for each filtrate jet also increased in order to fluidize the media bed. With the new design, the horizontal distance from the central manifold to the periphery is constant with depth and with equal spacing of the orifices or nozzles on the central manifold, each jet from the orifices fluidizes an equivalent sized zone of media. To fluidize deeper media beds for a given filter diameter requires simply extending the length of the central manifold and adding more orifices, each with equivalent flow and pressure. The flow required to fluidize a given diameter of filter bed increases linearly with depth while pressure remains essentially constant with the radial flow design. With the previous downflow design, pressure and flow requirements increased with depth, therefore increasing energy costs for operation.
Further, it was desirable to develop simple mechanisms to rotate the central manifold and control the speed of rotation. Speed control is relatively important in this design since a period of time is required for the horizontal jets to penetrate the media bed and totally fluidize a given zone all the way to the periphery of the bed.
Rotational speed controls developed for some previous downflow designs relied on expensive low speed gear motors and relatively complex mechanical configurations. Given the often corrosive, environments in which the filters operate (often salt water) the costs were significant. Significant maintenance was required and mechanical failures were more frequent than desired. The goal was therefore to develop a simple design which would be inexpensive and dependable.
Accordingly, in the present design, jets of filtrate from the vertical rotating central manifold fluidize an arcuately narrow vertical zone of media pellets in a radial direction from the centre to the periphery of the filter. The pressurized jets of filtrate work their way through the media bed until the pellets in a narrow vertical zone are completely fluidized. Fluidization of the zone of media from the centre to the periphery however requires several seconds.
The viscosity of the media is very low in the fluidized zone relative to the adjacent non-fluidized zone. The injector system of the invention utilizes this viscosity differential and the time lag for fluidization of a given zone, as a basis for rotational speed control.
A second vertically extending manifold, a thrust injector or thrust manifold, is located at the outer perimeter of the filter bed and is preferably connected to the vertical central manifold by horizontal support manifolds which are above and below the media bed. The thrust manifold is offset so that the horizontally directed filtrate jets from the central manifold are directed ahead of it. Orifices are located down the side of the thrust manifold and are oriented horizontally perpendicular to the central manifold orifices, that is, oriented generally in a tangential direction to the bed of media. Thrust created by the pumped filtrate emerging from the thrust manifold orifices pushes the thrust manifold forward into the low viscosity, fluidized zone created by the jets from the central manifold. The central manifold is therefore continually creating a low viscosity zone rotationally in front of the thrust manifold, so very limited thrust is required to move the vertical thrust manifold ahead. The viscosity of the unfluidized bed of media will not allow the thrust manifold to move forward beyond the zone fluidized by the jets from the central manifold. Since the two manifolds are physically connected by the support manifolds and in fluid communication with each other, a positive feedback control is established and the injection system rotational speed is therefore self governed and ensures that the thrust manifold cannot rotate unless complete fluidization of the zone in front of the thrust manifold by the jets from the central manifold is achieved from the centre to the periphery of the bed. With each complete revolution of the manifold through the pelleted media, the entire bed is thoroughly fluidized and the filtrate is uniformly distributed to all biofilm surfaces in the filter media bed.
Filtrate flow rates can be increased substantially if desired and additional thrust manifolds can be added to the central manifold. The distance that a pressurized jet of filtrate can effectively penetrate a bed of media is limited, for example, approximately 0.5 m, before the energy is significantly dissipated. To fluidize wider diameter beds of media, the horizontal support manifolds can be extended by additional support manifolds and additional or secondary vertical injectors or manifolds can be added between the additional support manifolds at intervals, for example, at intervals of approximately 0.5 m. These vertical secondary manifolds are similar in design to the central manifold. However, each of the secondary manifolds is offset from the one immediately inward thereof in order for the filtrate jets of the radially inward manifold to fluidize the arcuate zone in front of the manifold and thus enable it to move forward. Only the radially outermost manifold need be of the thrust manifold configuration since the maximum torque is achieved by providing thrust at the inner periphery of the tank.
The new injector system could also be potentially applied to larger filter bodies of circular or other polygonal shapes. A number of injector units could be supported on a frame above a bed of media and the injectors would each act to fluidize overlapping cells of media. A pipe manifold system would be used to uniformly distribute the filtrate to each of the multiple injector heads.
Further, it will be apparent that the new injector system can be retrofitted to existing bioreactor systems. A manifold structure comprising the central manifold with radially directed openings in association with an offset thrust manifold suitably supported and capable of ejecting filtrate in accordance with the above, can be easily incorporated into an existing bioreactor tank with minimal piping restructuring.
The disclosed method of injecting the filtrate is very efficient and minimizes the flow requirements in comparison with other and conventional fluidization techniques which fluidize the entire bed and require very high flow rates with large pumping rates and energy consumption.
As with the previous bioreactor designs, solids consisting of excess sheared biofilm and fine particulates settle and are flushed daily from the system via a bottom drain valve. This flushing is the only required maintenance for the bioreactor as it is otherwise self-cleaning.
A gear motor driven, vertical injector manifold represents an alternative to the water powered design. This option is a useful alternative for filter applications when filtrate flows and pressures are insufficient to provide adequate thrust to rotate the manifold apparatus. Additionally, as the filter tank diameter increases and the thrust manifold is positioned further from the central manifold, the torque increases for a given flow and pressure of filtrate. With small diameter filters (less than 50 cm.), therefore, in applications with relatively low flows of filtrate, there may be inadequate power to rotate the manifold. In such applications, a simplified motor driven injector manifold design, consisting of the central injection manifold without the thrust manifold and upper and lower support manifolds is a viable solution. Since the central manifold rotates around the center axis of the filter, there is very little friction involved since there is no apparatus actually moving through the viscous filter media. Therefore, a small, very low torque gear motor would be sufficient.
Accordingly, the invention in one broad aspect provides apparatus for use in association with a bioreactor tank having a bed of media pellets to be fluidized and for treating filtrate in the tank through biofilm adhering to the pellets. The apparatus including a vertically elongate central manifold having a plurality of openings longitudinally spaced along its length, the openings in the central manifold being substantially axially aligned and included in a vertical plane extending radially outwardly of the central manifold. The central manifold includes conduit means by which filtrate can be conveyed to and out of the openings. Means is provided for mounting the central manifold for rotation within a bioreactor tank having an inner periphery of wall. Means is also provided for rotating the central manifold at a predetermined speed when the central manifold is mounted in the bioreactor tank. Thus, when the central manifold is in operative association with the tank, filtrate communicated to the manifold openings under pressure is ejected substantially horizontally from the manifold openings in the plane to fluidize pellets cyclically in an arcuately narrow vertical zone extending between the central manifold and the peripheral wall of the tank as the central manifold is rotated.
Another aspect of the invention provides a method of treating filtrate in a bioreactor apparatus having a bed of buoyant media pellets floating within the filtrate to be processed in a tank having a peripheral wall for containing the filtrate and the bed of media pellets. The method includes the steps of providing a rotatably vertically supported central manifold within the tank, the central manifold having a plurality of longitudinally spaced radially directed openings intermediate its ends, providing means for rotation of the central manifold, feeding filtrate to the central manifold and out the openings while rotating the central manifold whereby a plane of filtrate is ejected from the openings to cyclically fluidize an arcuately narrow vertical zone of pellet media outwardly of the central manifold between the central manifold and the peripheral wall of the tank.
More preferably, the apparatus and method include providing a thrust manifold adjacent to the inner peripheral wall of the tank which is connected with and/or fluid communication with the central manifold and is designed with openings through which filtrate is forced but in a tangential direction, to the tank wall, to cause rotation of the central manifold in a self controlled manner.
With respect to the media pellets, applicant has found that pellets having certain physical parameters and optical dimensional ranges are to be preferred for the most efficient operation of the bioreactor herein. A simple configuration of a pellet is preferable, which can be manufactured in a one step, low cost extrusion process, the extruded length with appropriate grooves/ridges being sliced to produce the final pellets. Although pellets fabricated by combinations of other manufacturing processes, such as injection or extrusion, combined with secondary stamping or roll forming of surface configurations, are recognized as possible, designs of pellets which are compatible with one step extrusion are more cost effective to fabricate. Nevertheless, the pellet design is not a random design but is engineered to very specific criteria to be described herein.
Accordingly, a still further aspect of the invention comprehends a media pellet for use with a bioreactor system wherein a plurality of pellets are within a filtrate to be treated. Each pellet has specific gravity of from 0.91 to 0.95 with at least one surface having ridges and grooves, the grooves being approximately 1 mm in width and 1 mm in depth, the ridges being greater than 1 mm in width to prevent interlocking with other like pellets and the pellet has unit weight in the range of 0.05-0.07 gms.